cmd/tailscaled, logtail: share link monitor from wgengine to logtail

Part of overall effort to clean up, unify, use link monitoring more,
and make Tailscale quieter when all networks are down. This is especially
bad on macOS where we can get killed for not being polite it seems.
(But we should be polite in any case)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -36,6 +36,7 @@
 	"tailscale.com/version"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/magicsock"
+	"tailscale.com/wgengine/monitor"
 	"tailscale.com/wgengine/netstack"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/tstun"
@@ -195,6 +196,12 @@ func run() error {
 		go runDebugServer(debugMux, args.debug)
 	}
 
+	linkMon, err := monitor.New(logf)
+	if err != nil {
+		log.Fatalf("creating link monitor: %v", err)
+	}
+	pol.Logtail.SetLinkMonitor(linkMon)
+
 	var socksListener net.Listener
 	if args.socksAddr != "" {
 		var err error
@@ -205,7 +212,8 @@ func run() error {
 	}
 
 	conf := wgengine.Config{
-		ListenPort: args.port,
+		ListenPort:  args.port,
+		LinkMonitor: linkMon,
 	}
 	if args.tunname == "userspace-networking" {
 		conf.TUN = tstun.NewFakeTUN()

logtail/logtail.go

@@ -19,6 +19,7 @@
 
 	"tailscale.com/logtail/backoff"
 	tslogger "tailscale.com/types/logger"
+	"tailscale.com/wgengine/monitor"
 )
 
 // DefaultHost is the default host name to upload logs to when
@@ -106,6 +107,7 @@ type Logger struct {
 	url            string
 	lowMem         bool
 	skipClientTime bool
+	linkMonitor    *monitor.Mon
 	buffer         Buffer
 	sent           chan struct{}   // signal to speed up drain
 	drainLogs      <-chan struct{} // if non-nil, external signal to attempt a drain
@@ -128,6 +130,14 @@ func (l *Logger) SetVerbosityLevel(level int) {
 	l.stderrLevel = level
 }
 
+// SetLinkMonitor sets the optional the link monitor.
+//
+// It should not be changed concurrently with log writes and should
+// only be set once.
+func (l *Logger) SetLinkMonitor(lm *monitor.Mon) {
+	l.linkMonitor = lm
+}
+
 // Shutdown gracefully shuts down the logger while completing any
 // remaining uploads.
 //